Optical semiconductor devices that combine an optical semiconductor element such as an LED (light-emitting diode) with a phosphor are advantageous in terms of, for instance, high energy efficiency and long life. Accordingly, there is growing demand for such optical semiconductor devices in all manner of applications including outdoor displays, portable liquid crystal backlights and vehicle on-board applications. This trend has been accompanied by problems such as rising junction temperatures on account of greater heat output in the elements, or deterioration of element materials caused by the increase in direct light energy, as the luminance of such LED devices grows ever higher. The development of element materials having resistance to thermal degradation and light degradation has become thus an issue in recent years. Japanese Unexamined Patent Application Publication No. 2006-140207 discloses an optical semiconductor element mounting substrate having excellent reflectance characteristics after heat resistance testing.
When manufacturing a substrate by transfer molding using the thermosetting resin composition for light reflection disclosed in the above patent document, however, resin contamination occurs readily in that the resin composition seeps through gaps between the upper and lower dies of the mold during molding. When resin contamination occurs during heat molding, the contamination gets into the opening (recess) in the substrate that constitutes the optical semiconductor element mounting region, and hinders thereby mounting of the optical semiconductor element. Even if the optical semiconductor element can be mounted in the opening, such resin contamination obstructs electrical connection of the optical semiconductor element and metal wiring, effected by way of wire bonding or the like. That is, resin contamination is undesirable in that it impairs workability in element mounting and wire bonding during manufacture of a semiconductor element. When resin contamination is present at the opening of a substrate, a step of removing resin contamination is normally added to the manufacturing processes of optical semiconductor element mounting substrates, in such a manner so as to prevent the above-described hindrances during semiconductor element manufacturing. Owing to the cost and manufacturing time loss incurred, however, such removal step is still open to improvement.